The present invention relates to a hydrodynamic type bearing unit. This bearing unit is suitable for use as a spindle support particularly for the spindle motors of information equipment (for example, the spindle motors of magnetic disk devices, such as HDDs and FDDs, optical disk devices, such as CD-ROMs and DVD-ROMs, and magneto-optical disk devices, such as MDs and MOs, or the polygon scanner motors of laser beam printers (LBPs)).
Required of the spindle motors of said various items of information equipment are high rotation accuracy, high speed, low cost, low noise, etc. One of the components which determine these required items of performance is a bearing which supports the spindle of said motor. A conventional hydrodynamic type bearing exhibiting superior characteristics that satisfy the required performance criteria is known.
FIG. 5 shows an example of this kind of spindle motor constructed such that a shaft member 112 (composed of a shaft 112a and a thrust disk 112b which serves as a flange portion when mounted on the shaft 112a) rotatably supported by a bearing unit 111 is driven for rotation by the exciting force produced between a motor stator Ms and a motor rotor Mr. The bearing unit 111 is composed by disposing the shaft member 112 on the inner periphery of a cylindrical bearing member 114. The bearing unit 111 is provided with a radial bearing portion 115 for supporting the shaft member 112 in the radial direction and a thrust bearing portion 116 for supporting the thrust disk 112b in the thrust direction. Hydrodynamic type bearings having hydrodynamic pressure-producing grooves (hydrodynamic grooves) in the bearing surfaces are used as these bearing portions 115 and 116. The hydrodynamic grooves in the radial bearing portion 115 are formed, e.g., in the inner peripheral surface of the bearing member 114, while the hydrodynamic grooves in the thrust bearing portion 116 are formed, e.g., in the opposite end surfaces of the thrust disk 112b fixed to the lower end of the shaft 112a. 
When the shaft member 112 rotates, hydrodynamic oil films are formed in a bearing clearance Cr (defined between the outer peripheral surface of the shaft 112a and the inner peripheral surface of the bearing member 114) in the radial bearing portion 115 and bearing clearances Cs1 and Cs2 (defined between the opposite end surfaces of the thrust disk 112b and the surface of the bearing member 114 opposed thereto) in the thrust bearing portion 116, and the shaft member 112 is rotatably supported in the state of non-contact with respect to the bearing member 114.
In recent years, in said various items of information equipment, there has been a strong demand for weight reduction and thickness reduction. Particularly in HDDs, there has been a tendency for the range of use to expand from the use of the stationary type, such as desktop personal computers used so far, to the use of the portable type, such as mobile type PCs and digital cameras, such demand being particularly strong.
With the bearing unit 111 shown in FIG. 5, however, thickness reduction beyond the status quo has been limited because of the individual provision of the radial bearing portion 115 and the thrust bearing portion 116. Particularly for a use in which compatibility with PC cards is required, thickness reduction of HDDs is essential; however, with the construction in FIG. 5, it has been hard to take sufficient measures therefor.
Accordingly, objects of the invention are to provide a hydrodynamic type bearing unit which makes thickness reduction and weight reduction possible and which is produced at low cost, and to provide a method of producing said bearing unit at low cost and with high accuracy.
To achieve said objects, the invention provides a hydrodynamic type bearing unit comprising a shaft member and a bearing member opposed to an outer periphery of the shaft member via a bearing clearance, and supporting said shaft member without contact by means of the hydrodynamic pressure produced in said bearing clearance during the relative rotation between the shaft member and the bearing member, wherein said shaft member is composed of a plurality of parts and said bearing clearance is composed of a first clearance on one axial side and a second clearance on the other axial side, said first and second clearances being mutually oppositely inclined against an axial direction.
By mutually oppositely inclining the first and second clearances in this manner, the support force components of the hydrodynamic pressures produced in the first and second clearances act in the radial direction and opposite thrust directions, so that radial loads and opposite thrust loads can be stably held in the same manner as in a conventional bearing unit having radial and thrust bearing portions. Furthermore, unlike the conventional article, since the two bearing portions are integrally constructed, the thickness reduction and weight reduction of the bearing unit is attained.
The first and second clearances are inclined such that they are displaced greater toward the inner diameter as the bearing center (the axial center of the bearing) is approached, whereby the loading capacity for the moment around the axis can be increased.
Various items of information equipment have a strong demand for weight reduction and thickness reduction. Particularly in HDDs, there has been a tendency for the range of use to expand from the use of the stationary type, such as desktop personal computers used so far, to the use of the portable type, such as mobile type PCs and digital cameras, such demand being particularly strong.
According to the invention, the width between the first and second clearances is made adjustable by managing the axial relative positions of the first and second shaft members and the axial alignment. In the conventional article, since the thrust bearing portion is formed within the unit, it has been difficult to maintain the bearing clearance with high accuracy, whereas according to the invention, since the width between the first and second clearances can be managed from outside the unit by adjusting the axial relative positions of the first and second shaft members, it is possible to obtain bearing clearances of high accuracy. In this case, if a guide portion is provided between the first and second shaft members for axially guiding either of the members, then the guiding action of the guide portion allows the axial relative positions of the first and second shaft members to be adjusted while maintaining the axial alignment between the first and second shaft members, so that accurate bearing clearances can be easily obtained.
By making the bearing member of sintered metal retaining oil therein, it is possible to attain pumping balance between the first and second clearances to increase the operating stability.
The hydrodynamic type bearing units described above can be produced by disposing a clearance defining member between the first shaft member and the bearing member and intimately contacting said clearance defining member with the first shaft member and bearing member, intimately contacting the second shaft member with the bearing member and fixing them together in this state, and removing said clearance defining member to define first and second clearances. In this case, the accuracies of the first and second clearances depend on the accuracy (thickness) of the clearance defining member rather than on the accuracy of such single parts as the shaft member and bearing member; thus, bearing clearances of high accuracy can be obtained at low cost without being influenced by the accuracy of the single parts.
In this production process, it is preferable that the clearance defining member be made of resin material and that the removal of the clearance defining member be effected by a solvent.
A lightweight thickness-reduced spindle motor (disk device spindle motor, LBP polygon scanner motor or the like) for information equipment is constructed, wherein the spindle of information equipment is rotatably supported by a hydrodynamic type bearing unit described above.